(1) Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device with wide viewing angle characteristics of the active matrix system using thin-film transistors.
(2) Description of the Prior Art
Active matrix liquid crystal display devices using active elements as represented by thin-film transistors (TFT) have now been widely used as display terminals of OA equipment since they are light in weight and have a high picture quality equivalent to that of cathode-ray tubes.
The display system of the liquid crystal display devices can be roughly divided into two. One is a system in which liquid crystals are sandwiched by two substrates having transparent electrodes and are driven by a voltage applied to the transparent electrodes, and light incident upon the liquid crystals and transmitted through the transparent electrode is modulated to achieve display. All of the products that are now available are based upon this system. Another one is a system in which liquid crystals are driven by an electric field which is nearly in parallel with the surface of a substrate between two electrodes that are formed on the same substrate, and light incident upon the liquid crystals through a gap between the two electrodes is modulated to provide display. Though no product which is based upon this system has yet been provided, it has a feature of a very wide viewing angle and a promising art in connection with active matrix liquid crystal display devices.
Features of the latter system have been disclosed in, for example, Japanese Patent Laid-Open No. 505247/1993, Japanese Patent Publication No. 21907/1988 and Japanese Patent Laid-Open No. 160878/1994.
A first problem that is to be solved by the present invention will be described below.
In a conventional liquid crystal display device of the latter system, an electric field which is substantially in parallel with the surface of the substrate is generated via thin-film electrodes having thicknesses of about several thousand angstroms, making it difficult to effectively generate the electric field in the liquid crystal layer compared with the former system.
Therefore, an electric field stronger than that of the former-system must be generated between the electrodes, resulting in an increase in consumption of electric power and making it necessary to employ, as a driver unit, an LSI having an increased breakdown voltage.
A second problem is that in the former system, a metal material having good light-shielding property has been used as a black matrix (light-shielding film) that covers the portions where undesired light passes through. When this metal material is used for the latter system, however, the electric field between the electrodes is absorbed by the black matrix, making it difficult to generate an effective electric field between the electrodes.
A third problem is that in the former system, the electric field from a video signal line is absorbed by a counter electrode that is formed on nearly the whole surface of a substrate opposed to the substrate on which the video signal line is formed, and the electric field formed by the video signal line does not affect the electric field established between the electrodes. In the latter system in which no electrode exists on the substrate opposed to the substrate on which the video signal line is formed, however, the electric field formed by the video signal line affects the electric field established between the electrodes, giving rise to the occurrence of crosstalk (particularly in the vertical direction of the screen) in which video information of other rows affect the display and, hence, appearance of striped image called vertical smear.
A fourth problem is that in the latter system in which the counter electrode can be formed linearly, the resistance of the counter electrode from the input end to the other end thereof becomes very larger than that of when the counter electrode is formed in a planar shape in the former system. Therefore, the counter voltage is not sufficiently fed to the terminal pixels, and the counter voltage is distorted by the video signals due to the capacitance at a portion where the counter voltage signal line intersects the video signal line, resulting in the occurrence of crosstalk (particularly, in the horizontal direction of the screen) and appearance of striped image called lateral smear.
A fifth problem is that in the latter conventional system in which the pixel electrode PX and the counter electrode CT are arranged on the same substrate, the opening area that contributes to the display decreases by the amount corresponding to the arrangement of the counter voltage signal line CL compared with that of the former conventional system.
Moreover, an increase in the number of intersecting points of the wirings arranged in the form of a matrix results in increased chances of short-circuiting among the wirings and in an increase in the parasitic capacitance among the signal lines, hindering smooth transfer of signals.
Besides, while the pixel electrode PX in the former system has a planar shape, the pixel electrode PX of the latter system has a narrow strip shape or a line shape, often causing pixels to become defective due to disconnection of the line.
A sixth problem is that the latter conventional system may employ an AC driving method to apply an AC voltage to the liquid crystal layer, e.g., an AC driving method which inverts a drive voltage applied to the liquid crystal layer after every horizontal scanning interval. In this case, when a pulse voltage is applied to the counter voltage signal line CL having a resistance R and a capacitance C, the pulse voltage is distorted. Hereinafter, the sixth problem will be described with reference to FIGS. 25 and 26.
FIG. 25 is a diagram of an equivalent circuit of a transmission passage for transmitting a drive voltage applied to the counter voltage signal line CL in a liquid crystal display device of the latter system, and FIG. 26 is a diagram showing waveforms of a drive voltage applied to the counter electrode CT at each of the points.
The transmission passage for transmitting a drive voltage applied to the counter electrode CT includes chiefly resistors 50 of the counter voltage signal line CL, a resistor 51 of a common bus line CB between a common voltage driver unit 52 and the counter voltage signal line CL, and storage capacitors 53 in the pixels. Therefore, when a liquid crystal layer is driven by an AC drive voltage, the waveform of the drive voltage (pulse voltage) fed to the counter electrode CT from the common voltage driver unit 52 of a common voltage generator and driver unit 103 is distorted.
As will be understood from a counter voltage waveform 54 at point D, a counter voltage waveform 55 at point E, a counter voltage waveform 56 at point F and a counter voltage waveform 57 at point G shown in FIG. 26, the distortion of waveform of the drive voltage fed to the counter electrode CT increases with an increase in the distance, from point D, to point G.
As a result, the electric field between the pixel electrode PX and the counter electrode CT in the pixels differs, irregular brightness (irregular display) occurs along the counter voltage signal line CL, and the display quality of the liquid crystal display panel is impaired.
This becomes a serious problem particularly when there is employed an AC drive system which inverts the drive voltage applied to the liquid crystal layer after every horizontal scanning interval.
When the counter voltage signal line CL is broken even at one place, furthermore, it is impossible to drive the liquid crystals since the drive voltage is no longer fed to the counter electrode CT of the pixels after the broken portion, impairing the display quality of the liquid crystal display panel.
According to the constitution of the prior art, furthermore, the thickness differs by a thickness of the signal line depending upon the portions where the video signal line DL and the scanning signal line GL are led out and the portions where such signal lines are not led out. Therefore, the gap becomes irregular in the whole panel impairing the display quality of the liquid crystal display panel.
The object of the present invention is to provide a liquid crystal display device of high picture quality that can be efficiently fabricated, that is powered on a low voltage and consumes small amounts of electric power, and has wide viewing characteristics as a result of solving the above-mentioned problems inherent in the prior art.
Description of representatives of the invention disclosed in this specification is as follows.
Means for solving the above-mentioned first to fourth problems will be described below.
Means 1.
A liquid crystal display device having a pair of substrates at least one of which is transparent, a liquid crystal layer sandwiched by the pair of substrates, a pixel electrode and a counter electrode formed between one of the substrates and the liquid crystal layer, in order to change the light transmission factor or the light reflection factor of the liquid crystals by use of an electric field which is established between the pixel electrode and the counter electrode and has a component in parallel with the one substrate, wherein the dielectric constant anisotropy xcex94∈ and the effective thickness deff of the liquid crystal layer, the width Wp of the pixel electrode in the lateral direction, the width Wc of the counter electrode in the lateral direction, and the gap L between the pixel electrode and the counter electrode satisfy the following relationships,
xcex94∈ greater than 0, 2.8 xcexcm less than deff less than 4.5 xcexcm,
1.2xc3x97deff less than Wp less than L/1.2, and 1.2xc3x97deff less than Wc less than L/1.2
Means 2.
A liquid crystal display device-having a pair of substrates at least one of which is transparent, a liquid crystal layer sandwiched by the pair of substrates, a pixel electrode and a counter electrode formed between the one of the substrates and the liquid crystal layer, in order to change the light transmission factor or the light reflection factor of the liquid crystals by use of an electric field which is established between the pixel electrode and the counter electrode and has a component in parallel with the one substrate, wherein the dielectric constant anisotropy xcex94∈ and the effective thickness deff of the liquid crystal layer, the width Wp of the pixel electrode in the lateral direction, the width Wc of the counter electrode in the lateral direction, and the gap L between the pixel electrode and the counter electrode satisfy the following relationships,
xcex94∈ less than 0, 4.2 xcexcm less than deff less than 8.0 xcexcm 1.2xc3x97deff less than Wp less than L/1.2, and 1.2xc3x97deff less than Wc less than L/1.2
Means 3.
A liquid crystal display device having a pair of substrates at least one of which is transparent, a liquid crystal layer sandwiched by the pair of substrates, a pixel electrode and a counter electrode formed between one of the substrates and the liquid crystal layer, whereby pixels are formed such that the light transmission factor or the light reflection factor of the liquid crystals is changed by an electric field which is established between the pixel electrode and the counter electrode and has a component in parallel with the one substrate, and forming a black matrix surrounding the pixels on a plane, wherein the black matrix is made of an insulating material.
Means 4.
A liquid crystal display device having a pair of substrates at least one of which is transparent, a liquid crystal layer sandwiched by the pair of substrates, a video signal line, a drain electrode, a pixel electrode to which is applied a video signal from the video signal line via the drain electrode and a thin film transistor, and a counter electrode, that are formed between one of the substrates and the liquid crystal layer, whereby pixels are formed such that the light transmission factor or the light reflection factor of the liquid crystals is changed by an electric field which is established between the pixel electrode and the counter electrode and has a component in parallel with the one substrate, wherein the counter electrodes of adjacent pixels are positioned on both sides of the video signal line, and the widths of the counter electrodes are not smaller than one-half the width of the video signal line.
Means 5.
In the means 4, the counter electrode is made of an anodizable metal, and a self-anodized film of this metal covers the counter electrode.
Means 6.
In the means 5, the counter electrode is made of aluminum.
Means 7.
In the means 5, the counter electrode is formed simultaneously with the gate signal line which is made of a metal layer having anodized surface.
Means 8.
A liquid crystal display device having a pair of substrates at least one of which is transparent, a liquid crystal layer sandwiched by the pair of substrates, a video signal line, a drain electrode, a pixel electrode to which is applied a video signal from the video signal line via the drain electrode and a thin film transistor, a gate signal line connected to a gate electrode which turns the thin film transistor on, and a counter electrode to which a counter voltage is applied via a counter voltage signal line, which are formed between one of the substrates and the liquid crystal layer, whereby pixels are formed such that the light transmission factor of the liquid crystals is changed by an electric field which is established between the pixel electrode and the counter electrode and has a component in parallel with the surface of the one substrate, wherein the gate signal line is made of a metal layer having anodized surface, and the counter voltage signal line is made of the same material as that of the gate signal line.
Means 9.
In the means 8, the counter voltage signal line is made of aluminum.
Means 10.
In the means 8, the counter voltage signal line and the gate signal line are formed through the same step.
Means 11.
A liquid crystal display device having a pair of substrates at least one of which is transparent, a liquid crystal layer sandwiched by the pair of substrates, a video signal line, a drain electrode, a pixel electrode to which is applied a video signal from the video signal line via the drain electrode and a thin film transistor, a gate electrode for turning the thin film transistor on, a counter electrode to which a counter voltage is applied via a counter voltage signal line, and a storage capacitor formed by superposing part of the pixel electrode on part of the counter voltage signal line via an interlayer insulating film, which are formed between one of the substrate and the liquid crystal layer, whereby pixels are formed such that the light transmission factor of the liquid crystals is changed by an electric field which is established between the pixel electrode and the counter electrode and has a component in parallel with the surface of the substrate, wherein the counter voltage signal line is made of aluminum having anodized surface.
Means 12.
A liquid crystal display device having a pair of substrates at least one of which is transparent, a liquid crystal layer sandwiched by the pair of substrates, a video signal line, a drain electrode, a pixel electrode to which is. applied a video signal from the video signal line via the drain electrode and a thin film transistor, a gate electrode for turning the thin film transistor on, and a counter electrode to which a counter voltage is applied via a counter voltage signal line, which are formed between the one substrate and the liquid crystal layer, whereby pixels are formed such that the light transmission factor of the liquid crystals is changed by an electric field which is established between the pixel electrode and the counter electrode and has a component in parallel with the substrate, wherein a common bus line is provided to connect in common the counter voltage signal lines of the plurality of pixels, and the common bus line has a multilayer structure of two or more conductive layers.
Means 13.
In the means 12, the common bus line is made up of a conductive layer of the same material as that of the gate electrode and a conductive layer of the same material as that of the video signal line, and these conductive layers are formed simultaneously with the formation of the gate electrode and the video signal line.
In order to solve the above-mentioned fifth problem according to the present invention, the counter voltage signal line and the drain electrode are used in common by two pixels adjacent to each other in the direction of column, and the storage capacitor is formed in a part of the counter voltage signal line.
Means 14.
A liquid crystal display device having a pair of substrates at least one of which is transparent, a liquid crystal layer sandwiched by the pair of substrates, a video signal line, a drain electrode, a pixel electrode to which is applied a video signal from the video signal line via the drain electrode and a thin film transistor, a gate electrode for turning the thin film transistor on, a counter electrode to which a. counter voltage is applied via a counter voltage signal line, and a storage capacitor formed by superposing part of the pixel electrode on part of the counter voltage signal line via an interlayer insulating film, which are formed between the one of the substrates and the liquid crystal layer, whereby pixels are formed such that the light transmission factor of the liquid crystals is changed by an electric field which is established between the pixel electrode and the counter electrode and has a component in parallel with the substrate, wherein the scanning signal lines connected to the gate electrodes and the counter voltage signal lines connected to the counter electrodes are arranged in parallel in a first direction of the plurality of pixels that are arranged in the form of a matrix, and the video signals connected to the drain electrodes are arranged in parallel in a second direction, and wherein the counter voltage signal line is used in common by two pixels adjacent to each other in the second direction.
Means 15.
In the means 14, the gate electrodes, scanning signal lines and thin-film transistor elements of the two pixels adjacent to each other in the direction of column are so arranged as to be opposed to each other, the drain electrodes are used in common by the two pixels, and the wirings from the drain electrodes to the video signal lines are arranged between the opposing scanning signal lines.
Means 16.
In the means 15, the thin-film transistors are formed along the scanning signal line in such a way that the plurality of the thin-film transistors are connected to the pixel electrodes in a pixel;
Means for solving the above-mentioned sixth problem will be described below.
Means 17.
A liquid crystal display device having a pair of substrates at least one of which is transparent, a liquid crystal layer sandwiched by the pair of substrates, a video signal line, a drain electrode, a pixel electrode to which is applied a video signal from the video signal line via the drain electrode and a thin film transistor, a gate signal line connected to a gate electrode for turning the thin film transistor on, and a counter electrode to which a counter voltage is applied via a counter voltage signal line, which are formed between one of the substrates and the liquid crystal layer, whereby pixels are formed such that the light transmission factor of the liquid crystals is changed by an electric field which is established between the pixel electrode and the counter electrode and has a component in parallel with the substrate, wherein the ends on both sides of the counter voltage signal line are connected to a common bus line and are, further, connected to a common voltage generating and driving means.
Means 18.
In the means 17, thickness adjustment patterns are provided on the non-display area of one of the substrates, and the thickness adjustment patterns are made of the same material and have the same thickness as that of the common bus line.
Means 19.
In the means 17 or 18, a thickness adjustment film made of the same material and having the same thickness as that of the gate electrode is provided over or under the common bus line in the non-intersecting areas except the areas where the common bus line intersects the gate signal line or the video signal line and except the areas where the common bus line is connected to the counter voltage signal line.
Means 20.
In the means 17 or 18, a thickness adjustment film made of the same material and having the same thickness as that of the drain electrode is provided over or under the common bus line in the non-intersecting areas except the areas where the common bus line intersects the gate signal line or the video signal line and except the areas where the common bus line is connected to the counter voltage signal line.
According to the constitution of means 1, when a liquid crystal composition having positive dielectric constant anisotropy is used, it is possible to obtain a transmission characteristic little depending upon the wavelength in the birefringence mode, i.e., to obtain good white display, and to apply an electric field component to the liquid crystal layer in parallel with the substrate, which is much stronger than the component of the electric field in the direction perpendicular to the substrate. It is therefore possible to utilize the most efficient transmission state enabling the voltage between the pixel electrode and the counter electrode to be efficiently converted into the component in the direction in parallel with the substrate without raising the voltage for driving the liquid crystal.
According to the constitution of means 2, when a liquid crystal composition having negative dielectric constant anisotropy is used, it is possible to obtain a transmission characteristic little depending upon the wavelength in the birefringence mode, i.e., to obtain good white display, and to apply an electric field component to the liquid crystal layer in parallel with the substrate, which is much stronger than an electric field component in the direction perpendicular to the substrate. It is therefore possible to utilize the most efficient transmission state enabling the voltage between the pixel electrode and the counter-electrode to be efficiently converted into the one in the direction in parallel with the substrate without raising the voltage for driving the liquid crystal.
According to the constitution of means 3, it is possible to eliminate the effect upon the electric field between the pixel electrode and the counter electrode because the black matrix is constituted by an insulating material. The black matrix cuts off the effect upon the electric field between the pixel electrode and the counter electrode enabling the distance to be decreased among the electrodes. It is therefore allowed to increase the aperture ratio, and to effectively apply an electric field component in parallel with the surface of the substrate to the liquid crystal layer without raising the voltage for driving the liquid crystals.
According to the constitution of means 4, the lines of electric force from the video signal line can be absorbed by the counter electrodes that are located on both sides thereof, making it possible to prevent the occurrence of so-called crosstalk. In this case, the lines of electric force from the video signal line are equally divided by the counter electrodes on both sides each by 50%; i.e., 100% of the lines of electric force are absorbed in total.
According to the constitution of means 5, short-circuiting is prevented from occurring even when the counter electrodes on both sides are brought as close to each other as possible or even when they are arranged to intersect with the video signal line. This makes it possible to increase the aperture ratio.
According to the constitution of means 6, the counter electrodes have a small resistance. Therefore, a nearly uniform and stable current flows through the counter electrodes, and the counter voltage is sufficiently transmitted even to the pixels at the terminals, making it possible to further heighten the effect of means 4.
According to the constitution of means 7, it is possible to obtain the effect of means 5 without increasing the number of steps of production.
According to the constitution of means 8, it is possible to decrease the probability of short-circuiting at the areas where the counter voltage signal line intersects the video signal line.
According to the constitution of means 9, the counter voltage signal line has a small resistance and a nearly uniform and stable current flows through each of the counter electrodes. Therefore, a counter voltage is sufficiently transmitted to the pixels even at the terminals making it possible to achieve uniform pixel display.
According to the constitution of means 10, the effect of means 7 is gained without increasing the number of production steps.
According to the constitution of means 11, the electrode of the lower side formed via an interlayer insulating film is made of aluminum having an anodized surface, making it possible to form a storage capacitor little permitting the occurrence of troubles that are caused by point defects due to so-called whiskers.
According to the constitution of means 12, it is possible to decrease the resistance without increasing the width of the common bus line and to apply a sufficiently large voltage up to the ends of the counter electrodes. This makes it possible to decrease crosstalk (particularly, crosstalk in the horizontal direction of the screen) that is caused by distortion of the counter voltage according to the video signals.
According to the constitution of means 13, it is possible to obtain the effect of means 12 without increasing the number of steps of production.
According to the constitution of means 14, the gate signal lines GL connected to the gate electrodes GT and the counter voltage signal lines CL connected to the counter electrodes CT are arranged in parallel in the direction of row of a plurality of pixels arranged in the form of a matrix, the counter voltage signal line CL is used in common by the two pixels adjacent to each other in the direction of column, and the video signal lines DL connected to the drain electrodes SD2 are arranged in parallel in the direction of column, in order to decrease the parasitic capacitance among the wirings., to increase the production yield, to ensure the openings in the pixels and to decrease the resistances of the counter voltage signal lines CL.
According to the constitution of means 15, the gate electrodes GT, gate signal lines GL and thin-film transistor elements of two pixels adjacent to each other in the direction-of column in the constitution of means 14 are so arranged as to be opposed to each other, the drain electrode SD2 is used in common by the two pixels, and the wiring from the drain electrode SD2 to the video signal line DL is arranged between the opposing gate signal lines GL, in order to decrease the parasitic capacitance among the wirings, to increase the production yield, to ensure the openings in the pixels and to decrease the resistances of the counter voltage signal lines CL.
According to the constitution of means 16, the thin-film transistors are formed along the gate signal line GL in such a way that the plurality of thin-film transistors are connected to the pixel electrodes PX in one pixel of the constitution of means 15, in order to decrease the parasitic capacitances among the wirings, to increase the production yield, to ensure the openings in the pixels and to decrease the resistances of the counter voltage signal lines CL.
According to the constitution of means 17 to 20, both ends of the counter voltage signal lines CL are connected to the common bus line CB which has a resistance smaller than the resistances of the counter voltage signal lines CL. It is therefore possible to decrease distortion in the waveform of the drive voltage applied to the counter electrodes CT from the common voltage driver unit 52, to uniformalize the electric field intensity between the pixel electrode PX and the counter electrode CT in each pixel in the panel, and to decrease irregularity in the brightness that occurs along the counter voltage signal line CL.
Even in case the counter voltage signal line CL is broken at a portion, the common voltage is supplied from both ends of the counter voltage signal line CL to drive liquid crystal of the pixels. Unlike the prior art, therefore, the liquid crystal of pixels after the broken portion can be driven, and the quality of display is not impaired.
Furthermore, the area where the common bus line CB is arranged has the same cross-sectional structure as that of an area where the common bus line CB intersects the gate signal line GL or the video signal line DL, making it possible to decrease irregularity in the film thickness at the edges of the substrate where the common bus line CB is formed, to uniform the gap length between two substrates, and to decrease gap irregularity of the liquid crystal display device.
The foregoing and other objects, advantages, manner of operation and novel features of the present invention will be understood from the following detailed description when read in connection with the accompanying drawings.